The present invention relates to a capacitor based method of measuring dielectric characteristics of a liquid disposed between the electrodes of the capacitor.
Capacitance based sensors are commonly used to measure the quality of lubricating oils and other fluids used in engines and machinery. The capacitor sensors are positioned so that the fluid flows between the electrodes. A dielectric value of the fluid changes as the quality of the fluid degrades over time, temperature, use, and an introduction of other fluid types. For example, the dielectric value of engine lubricating oil tends to increase over time due to use, and will increase immediately if breached by water or glycol. This changing dielectric value results in a measurable change in the total capacitance of the capacitor sensor. When the dielectric value reaches a predetermined threshold then it is time to perform maintenance and replace the fluid.
Electric circuits used to measure the total capacitance of the capacitor sensor often operate by measuring a time required to charge/discharge the capacitor to/from a predetermined voltage threshold using a known voltage and a known series resistance. This method of measuring capacitance assumes that the fluid""s dielectric behavior is static. In other words, that the fluid behaves similar to a solid dielectric.
Lubricating oils and similar fluids often have different dielectric behavior than solid dielectric materials. Fluids often contain contaminants that exhibit dipole moments and electrostatic charges. These contaminants will move when exposed to the electric field created between charging electrodes within the capacitor. Dipole contaminants in homogeneous electric fields will move until they align with the field and the forces acting upon the two separate charges of the dipole cancel each other. Dipole contaminants in inhomogeneous electric field will experience electrostriction where the dipole contaminants are forced in a direction toward increasing field strength thus causing an elastic deformation of the fluid.
Ionized contaminants will move under the influence of homogeneous and inhomogeneous electric fields. Positively charged contaminants will move toward the negatively charged electrode, and negatively charged contaminants will move toward the positively charged electrode. The net result is a double-layer effect within the capacitor as the charged contaminants accumulate at the surface of the electrodes. Speed of the contaminant movement depends upon the electric field""s amplitude and rate of change, as well as the viscosity of the fluid through which the contaminants must move.
Dynamic contamination movement results in a time-dependent capacitance of the capacitor sensor. Conventional measuring techniques will result in varying capacitances depending upon the charge rate and predetermined voltage threshold used in the measurement. Short charge times do not give the contaminants sufficient time to reach equilibrium. Consequently, the measured capacitance is subject to random fluctuations as the contaminants move about between the electrodes. Long charge times allow the contaminants to stabilize. However, as the capacitor becomes fully charged, minor variations in the predetermined voltage can result in large changes in the measured charging time and thus the measured capacitance.
Not all contaminants will cause a change in the dielectric value. In some cases, the contaminant is another type of fluid having approximately the same dielectric value. For example, adding diesel fuel to clean engine lubricating oil causes little change in the measured dielectric constant. The quality of the fluid may be degraded by another fluid and yet conventional dielectric sensing techniques will not detect the degradation. The end result is that conventional capacitance based sensor measurements techniques lack accuracy under common conditions.
The present invention is a system and a method for measuring capacitance of a capacitor having a dielectric material disposed between the electrodes of the capacitor. The present invention utilizes a variable source in series with a resistor to induce a change in a current flowing through the capacitor. A sensor converts the resulting current flow into a current value readable by a processor. The processor records a plurality of current values at a plurality of times. An initial value is recorded corresponding to the induced change in the current. A leakage value is recorded after the current has exponentially decayed to approximate steady state. Multiple other current values are also recorded during the exponential decay.
An intermediate value is selected from among the multiple other current values based upon the initial value and the leakage value. The initial value, leakage value, intermediate value, a time between inducing the change in the current and determining the intermediate value, and a resistance of the resistor are then used to calculate the capacitance. This approach works well when the dielectric material causes a double-layer effect in the capacitor. This is because the capacitor is allowed to become almost fully charged and thus the electric field and the dielectric material within the electric field are given time to become stable.
In the preferred embodiment, the intermediate value is selected to be approximately equal to an ideal value set at 20 percent of the difference between the initial value and the leakage value. This places the intermediate value at a point on the exponential current decay where the capacitor has achieved a majority of its full charge and the measured current is changing at a moderate rate allowing for good analog to digital conversion. In alternative embodiments, the intermediate value is selected from a range of interest around the ideal value.
Once the capacitance has been calculated, a dielectric value for the fluid can be calculated from the capacitance and a known geometry of the capacitor sensor. The dielectric value provides a good trending indicator of the contamination levels within the fluid. Furthermore, a dielectric dissipation factor may also be calculated.
A conductivity of the dielectric material may also be calculated based upon the geometry of the capacitor, an applied voltage and the measured leakage value. This information is useful in trending changes to the dielectric material that do and do not cause a change in the dielectric value of the fluid.
A viscosity ratio of a fluid dielectric may be determined by measuring the leakage values at approximately 40 and 100 degrees Celsius and then calculating their ratio.
Accordingly, it is an object of the present invention to provide a system and method for measuring a capacitance of a capacitor having a dielectric disposed material between the electrodes of the capacitor where the dielectric creates a double-layer effect between the electrodes.
These and other objects, features and advantages will be readily apparent upon consideration of the following detailed description in conjunction with the accompanying drawings.